Five domestic shorthair farm cats with a median age of 3 years (range, 2 to 6.5 years) and mean ± SD body weight of 4.3 ± 0.6 kg (9.5 ± 1.3 lb) were evaluated at the Kansas State University Veterinary Health Center because of a sudden onset of tremors, obtundation, blindness, and dilated pupils. Less than 12 hours earlier, the owner had attempted to treat the cats for a suspected ear mite (Otodectes cynotis) infestation by aural administration of a dose of an ivermectin pastea intended for oral administration to horses (approx 22 mg/cat; half of the dose was administered into each ear canal). Physical examination abnormalities were limited to moderate amounts of debris in both ear canals of all 5 cats. Neurologic examination revealed generalized body tremors, mild obtundation, and bilateral ear twitching. Mydriasis in ambient light, diminished (sluggish and incomplete) pupillary light reflexes, and lack of menace response were also detected. All other findings of neuro-ophthalmic and physical examinations were unremarkable for all cats.
Complete ophthalmic examination, including slit-lamp biomicroscopy and indirect binocular ophthalmoscopy, was performed in all cats by the same veterinarian (JMM). In 4 of 5 cats, no abnormalities were detected in the anterior and posterior segments of the eyes. In the left eye of the fifth cat, several abnormalities were evident. Those in the anterior segment of the eye were mild and consisted of an incipient anterior cortical cataract with focal pigmentation on the anterior lens capsule. Fundic examination of the same eye revealed a large retinal tear and detachment with degeneration of the corpus vitreum and suspension of pigmented cells in the vitreous humor. These pathological changes were considered chronic in nature and unrelated to ivermectin toxicosis. No abnormalities were evident in the anterior segment or fundus of the right eye of that cat.
Pupils of all cats were dilated with topically applied 1% tropicamide solution,b a 20-minute dark adaptation period was provided, and ERGc then was performed. None of the cats were sedated, and 1 cat was deemed too fractious to allow testing. Retinal responses for each eye of the other 4 cats were assessed individually by use of a series of 8 bright light flashes; the mean of 8 responses was calculated by the ERG device to provide a single result for each eye. Retinal responses were abnormal in all cats (Figure 1). Responses were diminished in 7 eyes and totally eliminated in 1 eye (the left eye of the cat with retinal detachment). Median response values were 57.9 μV (range, 37.1 to 93.6 μV) for right eyes and 59.3 μV (range, 0.0 to 154.7 μV) for left eyes. Values > 100 μV were used to indicate healthy retinal function, in accordance with information provided by the manufacturer of the ERG device.
Because of financial constraints, toxicological testing for ivermectin was performed for only 2 cats. Blood samples were collected, and serum was harvested and submitted for analysis at an external laboratory.d
All cats were hospitalized for approximately 8 hours for observation. No supportive care was provided, and the cats were deemed stable for release into the owner's care at the end of the observation period. Cats were discharged to the owner with instructions that the owner monitor them for progression of neurologic signs (seizures and coma). During a follow-up telephone conversation 5 days after initial evaluation, the owner reported that vision and pupillary dilation had noticeably improved in all 5 cats. Other general neurologic signs of tremors and obtundation had reportedly resolved within 3 days after hospital discharge.
Results for toxicological testing revealed that ivermectin (450 μg/L and 610 μg/L) was detected in both submitted serum samples; no reference was available to indicate whether such concentrations were toxic in cats. Test results were negative for the presence of abamectin, moxidectin, and selamectin in both samples.
Approximately 1 month after initial evaluation, 4 of 5 cats were reexamined. The cat in which ERG could not be performed previously because of a fractious temperament was not reexamined. The owner reported that all cats recovered their vision. Findings of physical and neurologic examination were unremarkable for 3 of 4 cats. The fourth cat had a bilateral ear infection, and large amounts of malodorous, purulent discharge were evident. The owner declined diagnostic testing, such as cytologic evaluation and bacteriologic culture, to identify the cause of the infection. Consequently, thorough lavage of the cat's ear canals was performed, cefovecin sodiume (8 mg/kg [3.6 mg/lb], SC) was administered, and an ear cleanser for daily use was dispensed.
Complete ophthalmic examination revealed appropriate pupil size, unremarkable direct and consensual pupillary light reflexes, and unremarkable menace response in 3 of 4 cats; the cat with retinal detachment of the left eye remained blind (no menace response) in that eye. Previously detected fundic abnormalities in the left eye were also unchanged. For the right eye of that cat and both eyes of the other cats, findings of fundic examinations remained unremarkable. Electroretinography was repeated, and b-wave amplitudes were noticeably improved in 7 of 8 eyes (median value for right eye, 212.2 μV [range, 148.9 to 251.7 μV]; left eye, 151.0 [range, 0.0 to 225.2 μV]; Figure 1). Responses in the left eye of the cat with retinal detachment remained totally extinguished.
γ-Amino butyric acid
DuraMectin (1.87% ivermectin) paste, Durvet Inc, Blue Springs, Mo.
Tropicamide ophthalmic solution, Akorn Inc, Lake Forest, Ill.
RetinoGraphics BPM-100 system ERG/VEP, RetinoGraphics Inc, Norwalk, Conn.
California Animal Health and Food Safety Laboratory System, Davis, Calif.
Convenia, Zoetis Inc, Kalamazoo, Mich.
Acarexx (0.01% ivermectin) otic suspension, Boehringer Ingelheim Vetmedica Inc, St Joseph, Mo.
Ivomec (0.5% ivermectin) pour-on for cattle, Merial Ltd, Duluth, Ga.
5. Nelson OL, Carsten E, Bentjen SA, et al. Ivermectin toxicity in an Australian Shepherd dog with the MDR1 mutation associated with ivermectin sensitivity in Collies. J Vet Intern Med 2003; 17:354–356.
6. Lovell RA. Ivermectin and piperazine toxicoses in dogs and cats. Vet Clin North Am Small Anim Pract 1990; 20:453–468.
7. Kenny PJ, Vernau KM, Puschner B, et al. Retinopathy associated with ivermectin toxicosis in two dogs. J Am Vet Med Assoc 2008; 233:279–284.
8. Epstein SE, Hollingsworth SR. Ivermectin-induced blindness treated with intravenous lipid therapy in a dog. J Vet Emerg Crit Care (San Antonio) 2013; 23:58–62.
10. Paul AJ, Tranquilli WJ, Seward RL, et al. Clinical observations in Collies given ivermectin orally. Am J Vet Res 1987; 48:684–685.
11. Muhammad G, Abdul J, Khan MZ, et al. Use of neostigmine in massive ivermectin toxicity in cats. Vet Hum Toxicol 2004; 46:28–29.
13. Godber LM, Derksen FJ, Williams JF, et al. Ivermectin toxicosis in a neonatal foal. Aust Vet J 1995; 72:191–192.
14. Swor TM, Whittenburg JL, Chaffin MK. Ivermectin toxicosis in three adult horses. J Am Vet Med Assoc 2009; 235:558–562.
15. Norman TE, Chaffin MK, Norton PL, et al. Concurrent ivermectin and Solanum spp. toxicosis in a herd of horses. J Vet Intern Med 2012; 26:1439–1442.
16. Plummer CE, Kallberg ME, Ollivier FJ, et al. Suspected ivermectin toxicosis in a miniature mule foal causing blindness. Vet Ophthalmol 2006; 9:29–32.
17. Mealey KL, Bentjen SA, Gay JM, et al. Ivermectin sensitivity in Collies is associated with a deletion mutation of the mdr1 gene. Pharmacogenetics 2001; 11:727–733.
18. Longhofer SL, Daurio CP, Plue RE, et al. Ivermectin for the prevention of feline heartworm disease, in Proceedings. Heartworm Symp 1995;177–182.
19. Pagé N, de Jaham C, Paradis M. Observations on topical ivermectin in the treatment of otoacariosis, cheyletiellosis, and toxocariosis in cats. Can Vet J 2000; 41:773–776.
20. Yang XL. Characterization of receptors for glutamate and GABA in retinal neurons. Prog Neurobiol 2004; 73:127–150.
21. Ekesten B. Ophthalmic examination and diagnostics part 4: electrodiagnostic evaluation of vision. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary ophthalmology. 5th ed. Ames, Iowa: Blackwell, 2013;684–702.